1. Field of the Invention
The present invention relates generally to horizontal bandsaw machines in which a saw head assembly carrying a bandsaw blade is lowered during a cutting operation in order to feed the bandsaw blade into an underlying material to be cut, and more particularly pertains to methods and apparatus for controlling the feeding of the bandsaw blade in horizontal bandsaw machines.
2. Description of the Prior Art
Horizontal bandsaw machines conventionally comprise a saw head assembly in which a flexible endless bandsaw blade is trained around a pair of wheels so that it may perform cutting operations when the wheels are driven. The saw head assembly is so arranged as to be raised and lowered in relation to an underlying base supporting the material to be cut. This movement can occur around a hinge pin or along a single or plural guide posts in response to operation of a driving means such as a hydraulic motor. Thus, in cutting operations, the saw head assembly is lowered from the raised position toward the base so as to feed the bandsaw blade travelling therein into the material to be cut which has been placed on the base. Also, in most prevailing horizontal bandsaw machines, the saw head assembly is so arranged as to be raised by a hydraulic motor of a cylinder type when the hydraulic fluid is supplied thereinto and to be lowered by its own gravity when the hydraulic fluid is drained from the hydraulic motor.
In such horizontal bandsaw machines, the bandsaw blade is adjustably fed into the material to be cut at a higher or lower driven speed and at larger or smaller feeding force depending upon the nature of the material. Generally, the bandsaw blade is fed at a higher speed and a smaller feeding force when cutting normal easy-to-cut materials which are generally soft and brittle, and is fed at a lower speed and a larger feeding force when cutting difficult-to-cut materials which are generally hard and tough.
It is important to feed the bandsaw blade into the material to be cut so that the cutting action is performed always at a certain stated or uniform optimum cutting rate which is defined as the area (square millimeter) of cut per unit or time. Since many materials such as round bars and shape steels vary in cross-sectional length as cutting progresses therethrough, it is necessary to feed the bandsaw blade in correlation with this varying cross-sectional length of the material being cut so as to enable the bandsaw blade to operate continuously at a stated optimum cutting rate. Of course, if the bandsaw blade is not fed at an optimum cutting rate, it will work too hard and will become worn prematurely or otherwise it will work inefficiently.
For the purpose of performing cutting operations at a stated cutting rate according to the cross-sectional length of the material being cut, the bandsaw blade is conventionally arranged to be fed with a stated load or feeding force under the so-called load control or pressure control. In order to maintain the load or feeding force under the load control, the pressure of the hydraulic motor for the saw head assembly is controlled while the saw head assembly is lowered to feed the bandsaw blade into the material to be cut. The load control is intended to maintain the cutting resistance constant so as to feed the bandsaw blade at a constant cutting rate, since the cutting rate is generally proportional to the cutting resistance. Thus, under the conventional load control, the feeding speed of the bandsaw blade is controlled as a function of the cross-sectional cut length of the material to be cut so as to keep the cutting resistance constant, while the driven speed of the bandsaw blade is kept unchanged.
The conventional load control, however, is defective especially in that it is based on the premise that the cutting resistance per unit of feeding amount or feeding speed of the bandsaw blade which is defined as feeding speed/driven speed of the bandsaw blade, is always constant or uniform. In fact, the cutting resistance per unit of feeding amount of the bandsaw blade has a tendency to increase as the feeding amount decreases. Accordingly, when the feeding speed or amount is decreased in order to maintain the cutting resistance constant to respond to an increase of the cross-sectional length of the material being cut for instance, the cutting resistance per unit of feeding amount will have a tendency to further increase. The cutting resistance per unit of feeding amount will markedly increase, especially when cutting difficult-to-cut materials such as stainless steels which have to be cut at a low feeding speed or by a small feeding amount. Thus, the difficult-to-cut materials can not be cut at a uniform optimum cutting rate by the conventional load control in the horizontal machine, although the easy-to-cut materials which can be cut at a higher feeding speed namely by a larger feeding amount can be cut at a substantially uniform cutting rate under the conventional load control.